More and more biologists are beginning to realize that the history of life is not as determined by natural selection as they once thought. They are beginning to take to heart the idea that if you rewind and rerun the tape of life it will not turn out the same. A lot of the history is due to chance, luck, and accident.This week's issue of Science contains a paper by Brusatt et al. (2008) discussing the evolution of dinosaurs. The dinosaurs (Dinosaura) rose to prominence around 200 million years ago in what is often called an "adaptive radiation." The idea was that dinosaurs outcompeted all other land animals because they were better able to adapt to new environments.
At the beginning of this period, the main competitors were the crurotarsans, animals that resembled dinosaurs in many ways but which aren't classified as dinosaurs. Crocodiles are crurotarsans but all other families have gone extinct.
Recent work has revealed that the crurotarsans were as diverse and abundant as the dinosaurs 200 million years ago. The authors explain the problem ...The critical interval to consider is the Late Triassic, especially the Norian and Rhaetian (Fig. 1), a 28-million-year span between the CNEE [Carnian-Norian extinction event] and TJEE [Triassic-Jurassic extinction event]. The key "competitors" of the early dinosaurs were the crurotarsans, the "crocodile-line" archosaurs, which show a range of morphologies and adaptations during this time: long-snouted fish- and flesh-eating phytosaurs, armored herbivorous aetosaurs, and large to giant carnivorous "rauisuchians." The crurotarsans even replicated many dinosaurian body plans (large terrestrial predators; small swift predators; mid- to large-bodied low-browsing herbivores; agile bipedal herbivores). Several new discoveries show striking convergences between crurotarsans and dinosaurs (10), and many Triassic crurotarsans were previously erroneously identified as dinosaur ancestors (11) or even as true dinosaurs (12). Such morphological convergence suggests that dinosaurs and crurotarsans were exploiting similar resources in the Late Triassic. In some Norian faunas, crurotarsans were numerically more abundant than dinosaurs (3) and seem to have exploited a wider range of body plans. However, by the end of the Triassic all crurotarsans were extinct, save a few lineages of crocodylomorphs.
The key question is why the major dinosaur lineages survived the TJEE, ushering in the 135-million-year "age of dinosaurs," while most crurotarsan groups went extinct.
Brusatte et al. measured evolution rates, speciations, and morphological disparity for the two groups (crurotarsans and dinosaurs) before the Triassic-Jurassic extinction event. They found no evidence that the dinosaurs were evolving more quickly or were becoming better adapted than the crurotarsans. So, if crurotarsons were so successful why did they die out and why did the dinosaurs survive? Maybe the dinosaurs weren't better adapted, maybe they were just lucky.
The lead author of the study, Steve Brusatt, an M.Sc. student, puts it very well in the press release [Good Luck, Not Superiority, Gave Dinosaurs Their Edge, Study Of Crocodile Cousins Reveals].
"If we were standing in the Late Triassic, 210 million years ago or so, and had to bet on which group would eventually dominate ecosystems, all reasonable gamblers would go with the crurotarsans. There was no sign that dinosaurs were eventually going to succeed so why did they? The answer is two mass extinction events: the dinosaurs not only got lucky, but they got lucky twice.This paper is relevant for a number of reasons unrelated to the history of dinosaurs.
"They first weathered the storm during the Carnian-Norian event 228 million years ago, but so did the crurotarsans. In contrast, many other potential competitor groups went extinct. Then dinosaurs weathered a second, much bigger, storm 200 million years ago. This was the end Triassic extinction event, which was a sudden and catastrophic extinction caused by rapid climate change, possibly facilitated by an asteroid impact. Strangely, and suddenly, all crurotarsans except for a few lineages of crocodiles went extinct. On the other hand, the dinosaurs did not. They survived and then radiated in the Early Jurassic, and very quickly established themselves as the dominant vertebrate group on land across the world.
"Why did crurotarsans go extinct and not dinosaurs? We don't know the answer to that, but we suspect that it was nothing more than luck, plain and simple.
- It shows a trend away from pure adaptationist thinking toward consideration of other explanations (e.g., accident).
- It emphasizes the importance of understanding mass extinctions and incorporating them into macrevolutionary studies. The conclusions echo those of David Raup in his book Extinction: Bad Genes or Bad Luck [see Good Science Writers: David Raup].
- It is relevant to the discussion about convergent evolution—a feature of the history of life used by theistic evolution proponents to indicate that there's a plan to evolution [Convergence]. Note that there are many examples of convergence in the crurotarsan and dinosaur lineages. Similar examples with marsupials and placental mammals are used as evidence that evolution may have had a purpose in mind. But if we apply the same reasoning to crurotarsans and dinosaurs, the purpose becomes less obvious, since both groups eventually become extinct.
[Image Credit: The drawing of a crurotarsan archosaur is from the Palaeos website, specifically Archosauromorpha: Rauisuchiformes]
Brusatte, S.L., Benton, M.J., Ruta, M. and Lloyd, G.T. (2008) Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs. Science 321:1485-1488. [DOI: 10.1126/science.1161833]
S. L. Brusatte, M. J. Benton, M. Ruta, G. T. Lloyd (2008). Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs Science, 321 (5895), 1485-1488 DOI: 10.1126/science.1161833
15 comments :
I’m not sure I follow the argument with regards to NS (and strongly suspect it is a strawman). Nothing about NS discounts the role of chance. Is there anyone who seriously holds the view everything we see today is an inevitable outcome of a process started billions of years ago?
NS helps accounts for the present state, but cannot be used to predict the future. Thus, it strikes me as bizarre to imply that anyone’s predictions prior to the mass extinction about the winners would be based on how one views the importance of NS.
When there is a major environmental catastrophe, luck is probably the best word to describe the survivors, because no theory posits that the survivors pre-adapted and were ready to roll under the new conditions. Even the mythical “pure adaptationist” would not deny the role of chance, luck, accidents, etc., particularly at times of mass extinctions.
About the best I can see that follows from this paper (with respect to the topic of NS) is that it illustrates the point that evolution is not a process that can easily be predicted: it is a chaotic process subject to wildly different outcomes as a consequence of seemingly minor and hard to predict (and hard to discern, retrospectively) factors. And, at that, it is merely a restatement of something that is well accepted.
Mutations and survival are random events, and so, for example, whether a particular genetic mutation actually occurs, at the right time, and whether the individual that got it survives, and so on, are all contingencies that, had they not occurred, could have dramatically affected the outcome. Similarly, we know that the history of many species are a consequence of the rare occurrence of very improbable events (such as the rafting of the ancestor of new world monkeys to South America). Are you seriously suggesting that there are people who dispute this?
"Bad luck extinction vs. natural selection" is a fallacy, a false dichotomy.
In the modern evolutionary theory, natural selection is defined as the differential reproduction of genotypes (in a population of the same species).
So, natural selection is NOT differential success between species or taxa. When a species outcompetes its ecological "rival", that's not natural selection. When some species become extinct and others stay, that's not natural selection. Biologists are suppossed to know this.
divalent says,
When there is a major environmental catastrophe, luck is probably the best word to describe the survivors, because no theory posits that the survivors pre-adapted and were ready to roll under the new conditions. Even the mythical “pure adaptationist” would not deny the role of chance, luck, accidents, etc., particularly at times of mass extinctions.
The most common explanation for the survival of mammals and not dinosaurs is that the mammals were small and lived in burrows. They were also generalists.
Thus, the survival of mammals is not just due to luck but to the fact that they were better able to adapt to the conditions at the time of the asteroid impact.
You can see this explanation in many publications. Here's an example from the textbook Genetics, Paleontology, and Macroevolution by Jeffrey S. Levinton where he tries to account for the survivors of the Permian extinction event.
During the period just after the extinction, there perhaps was an interesting opportunity for expansion and adaptive diversification of species formerly rare, or even newly appearing. In many cases, these opportunities are taken up apparently by generalist taxa that survive the extinction and are of small body size and generalized morphology. (p. 422)
I think you'll find that a great many biologists believe that there were some differences between the survivors and the victims of mass extinctions—differences that enhanced their chances of surviving.
Larry (stating a position he is critical of): ... a great many biologists believe that there were some differences between the survivors and the victims of mass extinctions—differences that enhanced their chances of surviving.
Why do I get the funny feeling that you are trolling your own blog?
I can’t possibly see how this can be denied, or even seriously argued against, as a general statement of what went on in the aftermath of the KT impact. This is not a "pure adaptationist" position, it is a position that is based on evidence and reason. It has been said that nothing larger than a cat survived the KT event, and such an outcome is not likely to be the result of chance.
NS is working at all times, and will be particularly brutal when environmental change suddenly alters the landscape. In virtually an instant, the survival value of so many physical and behavioral attributes changed, and almost exclusively for the worse. Those species that survived were those that, on average, were fortunate to find themselves better adapted than other species to the new world environment.
Interesting research. Thanks for your article!
Of course extinction is not natural selection, at least not the kind that "directs" evolution, accumulating variation that would otherwise not accumulate, by sustained selective pressures. Extinctions are contingent episodes.
Since extinctions can change the history of evolution of life on earth, then we must acknowledge that this higher level, and not only natural selection within species, is also required to understand the evolution of life on earth.
As simple as that.It's not against natural selection, but it is not natural selection, either. Evolution is much more than natural selection.
divalent says,
I can’t possibly see how this can be denied, or even seriously argued against, as a general statement of what went on in the aftermath of the KT impact. This is not a "pure adaptationist" position, it is a position that is based on evidence and reason. It has been said that nothing larger than a cat survived the KT event, and such an outcome is not likely to be the result of chance.
If the argument is valid then one might expect it to apply to other mass extinctions. Brusatt et al, (2008) tested this by examining events after the Triassic-Jurassic extinction event.
They looked at two groups of reptiles and discovered that the surviving group (dinosaurs) was not smaller and/or more of a generalist than the other group that went extinct (crurotarsans).
The implication is that luck plays a big role and the survival of mammals may just be lucky and not due to any particular selective advantage they might have had in the aftermath of the asteroid impact.
Even though you don't see it, the tendency among biologists is to look for an adaptationist explanation whenever possible. That's why it seems so natural to you to explain the survival of mammals following the K/T event.
You've probably never seriously considered the possibility that their survival was just plain lucky. The so-called "evidence" that you refer to is just a bunch of correlations viewed in hindsight as rationalizations for an adaptationist explanation.
So I read this article with great interest, but it seems to me this article has a fundamental flaw (assuming I didn't miss something; I am not an expert in this area).
This article appears to have exclusively analyzed species that lived before the TJEE, but didn't look at the survivors.
So basically this study showed us that there is no difference in the gross measures of evolutionary fitness pre-TJEE; but what it doesn't show is if there were significant differences between the species that survive the TJEE and those who didn't.
Obviously I'm assuming things here - like only a subset of the pre-TJEE dino species survived (if most dino species survived, then my point is moot). Perhaps, if the survivors had been analyzed there would have been some identifiable trait that may have allowed them to survive. Or maybe it truly is just plain-dumb luck.
I just don't think the evidence is clear enough to say either way.
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On an unrelated note, one part of Larry's writeup that surprised me was the statement:
"convergent evolution —a feature of the history of life used by theistic evolution proponents to indicate that there's a plan to evolution"
This came as a shock - under the same logic wouldn't divergence be evidence against a plan (or suggest that lucifer was "leading astray" species from gods plans, or whatever). How do they account for the driving factors - mutation, for example - which are random?
But, are there any evolutionary biologists that don't acknowledge extinctions as required to "understand evolution of life on earth"?
I mean real (not straw) ones.
We have many poeple, biologists and not-so-biologists, that are dismissive of anything but natural selection...
And some poeple right here who are thinking extinction is just all natural selection, too
I wonder what conway-morris thinks about the rol of extinction.
Simon Conway Morris has an extreme, excentric, marginal view about evolution. He thinks "Mass extinctions may accelerate (maybe postpone), but they never cancel [evolutionary possibilities]". Being a extremist compared with the vast majority of evolution researchers, even he accepts some role of extinctions as slowing or accelerating factors.
And, this particular case of dinosaurs vs. crurotarsi doesn't challenge Conway Morris' ideas. We can see much convergent evolution between the two groups. There were already "dinosauroid" crurotarsi at the Triassic. The extinction of early dinos would have rendered (if we apply Conway Morris' thought) more "advanced" dinosauroid crurotarsi: diplodocus-like, tyrannosaur-like, hadrosaur-like, etc.
So I don't see why this new paper should shake any "selectionist", "adaptationist" or er... "ultra-convergentist" view of evolution.
They looked at two groups of reptiles and discovered that the surviving group (dinosaurs) was not smaller and/or more of a generalist than the other group that went extinct (crurotarsans).
The implication is that luck plays a big role and the survival of mammals may just be lucky and not due to any particular selective advantage they might have had in the aftermath of the asteroid impact.
I have not read the full article (always dangerous before commenting). Though I am ready to accept a defining role for contingency at multiple points in the history of life, I wonder to what extent we can say the data on the many ecological niches filled by crurotarsans necessarily implies with any precision how big a role luck played in their non-survival.
To be more specific, I wonder if there were any differences between crurotarsans and dinosaurs (the fact that both were generalists being a similarity) that might account to some degree for differential survival. For example, was there any difference in thermoregulation rendering the surviving group less sensitive to changes in climate?
This certainly doesn't relegate contingency to a minor role. When a majority of life on Earth vanishes, any survivors are lucky pretty much by definition. What I'm questioning is the extent to which we can say with any confidence that no factors other than pure luck played much of a role.
Jud asks,
To be more specific, I wonder if there were any differences between crurotarsans and dinosaurs (the fact that both were generalists being a similarity) that might account to some degree for differential survival. For example, was there any difference in thermoregulation rendering the surviving group less sensitive to changes in climate?
Why are you so anxious to find an adaptationist explanation?
Jud is not searching for an "adaptationist explanation". Adaptationism relies in the assumption that all or most traits are optimal adaptations (from Wikipedia, wich is pretty right in this... today) due to natural selection.
I don't see why looking to possible traits that cold help dinosaurs survive that massive extinction is "adaptationism".
Perhaps the "anxiety" is in the eye of the beholder ;-)
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